Ergonomists aim to prevent occupational injuries and to improve workers' well being and their performance. Many ergonomists feel the need to prove the benefits of their work. Next to evaluation with questionnaires, economical benefits of their work should be most welcome to demonstrate their positive contribution. However, mathematical models are scarce, and calculations are not as simple as they should like. In the Netherlands two studies were carried out to estimate the macro costs of (poor) working conditions. Macro costs are considered to be the total of all costs for employers, employees and costs to society. The latter ones are the costs for social insurances and the government. The first study focused at all workers nationwide, the second one at the building and construction industry only. These studies taught us a lot about the feasibilities and impossibilities to make calculations.